This application relates to magnetic devices having magnetic tunnel junctions such as magnetic memory devices.
A magnetic tunnel junction (MTJ) cell includes two magnetic electrode layers separated by a thin insulating layer known as a tunnel barrier. FIG. 1 shows one example of such an MTJ 100 where each magnetic layer 110 or 120 is in contact with an electrode layer 140 or 150. The electrode layers 140 and 150 electrically connect the magnetic layers 110 and 120 to a control circuit. The resistance across the MTJ 100 is determined by the relative orientation of the magnetization vectors of the magnetic layers 110 and 120. The magnetic layers 110 and 120 may be made of ferromagnetic (FM) alloys such as Fe, Co, Ni and the insulating barrier 130 may be made of an insulator material such as Al2O3 and MgO. Other suitable materials may also be used. The magnetization direction of one magnetic layer 120 of the MTJ 100 is pinned in a predetermined direction while the magnetization direction of other magnetic layer 110 is free to rotate under the influence of an external magnetic field or spin torque and is frequently referred to as a “free layer.” Pinning of the magnetic layer 110 of the two magnetic layers may be achieved through, e.g., the use of exchange bias with an antiferromagnetically ordered material such as PtMn, IrMn and others.
Such an MTJ cell may be used to construct a memory device such as a magnetic random access memory (MRAM). A MRAM may include multiple MTJ cells where a bit is stored in an MTJ by changing the relative magnetization state of the free magnetic layer with respect to the pinned magnetic layer. The stored bit can be read out by measuring the resistance of the cell which changes with the magnetization direction of the free layer relative to the pinned magnetic layer. In order for such an MRAM to have the characteristics of a non-volatile random access memory, the free layer must exhibit thermal stability against random fluctuations so that the orientation of the free layer is changed only when it is controlled to make such a change. This thermal stability can be achieved via the magnetic anisotropy using different methods, e.g., varying the bit size, shape, and crystalline anisotropy. Additional anisotropy can be obtained through magnetic coupling to other magnetic layers either through exchange or magnetic fields. Generally, the anisotropy causes a soft and hard axis to form in thin magnetic layers. The hard and soft axes are defined by the magnitude of the external energy, usually in the form of a magnetic field, needed to fully rotate (saturate) the direction of the magnetization in that direction, with the hard axis requiring a higher saturation magnetic field.
Various field switched MRAM devices use multiple or an array of magnetic memory cells and digit lines and bit lines to produce magnetic fields for addressing the magnetic memory cells. Each magnetic memory cell includes an MTJ, an isolation transistor, and a bit line and is physically located at the intersection of the digit and bit lines. The switching of the free layer occurs when DC write currents are applied to the bit and the digit lines. The resulting magnetic field at each memory cell, summed at the intersection of the current carrying lines, is set to be sufficient to rotate the free magnetic layer in the MTJ. The switching behavior can be described by the Stoner-Wohlfarth model of coherent rotation. See, E. C. Stoner and E. P. A. Wohlfarth, Phil. Trans. R. Soc. Lon. A 240 599 (1948).
FIG. 2 shows an example of the magnetic field properties of an MTJ for the switching and non-switching magnetic field phase space which is defined by an astroid curve along the hard and soft axes (labeled as “H hard” and “H easy”) of the free layer. When the applied magnetic field lies outside of the astroid curve, the bit is unstable and can switch. When the applied magnetic field lies inside the astroid curve, the in-plane magnetic coercivity of the free layer dominates and the magnetization direction of the free layer does not change with the applied magnetic field. However, distributions of in-plane anisotropy lead to variations in the astroid curve of the MTJ and such variations create a much smaller area of magnetic field phase space available to completely switch all the bits in the array without the unwanted switching the half-selected bit in the array.
FIG. 3 shows an example of a spin-transfer switched MRAM device 300 having an array of memory cells 310 where an MTJ 100 in each cell 310 is connected to an isolation/write transistor 320 and a bit line 330. Switching via spin-transfer occurs when a current, passing through a magnetic layer of the magnetic tunnel junction 100, becomes spin polarized and imparts a spin torque on the free layer of the MTJ. When a sufficient spin torque is applied to the free layer, the magnetization of the free layer can be switched between two opposite directions and accordingly the MTJ can be switched between the parallel and antiparallel states depending on the direction of the DC current. The isolation/write transistor 320 is connected to a DC source and controls the direction and magnitude of the DC current flowing through the MTJ 100 from the DC source. This control may be achieved by the relative voltages on the gate, source and drain of the transistor 320.
Such a spin transfer MRAM generally does not suffer from the half-select problem because switching occurs when the isolation/write transistor 320 is activated and a sufficient spin-polarized current is passed through the MTJ. In this manner, a single cell can be selected without disturbing any other cell in the array. See, e.g., U.S. Pat. No. 5,695,864 to Slonczewski.